Conventionally there have been proposals for wavelength modulation-type laser measuring devices that use the self-coupling effect of semiconductor lasers. (See Japanese Unexamined Patent Application Publication 2006-313080 (“JP '080”).) The structure of this laser measuring device is illustrated in FIG. 22. The laser measuring device in FIG. 22 includes a semiconductor laser 201 for emitting a laser beam at an object 210; a photodiode 202 for converting, into an electric signal, the optical output of the semiconductor laser 201; a lens 203 for focusing the beam from the semiconductor laser 201 and directing it toward the object 210, and for focusing the light that is reflected from the object 210 and allowing the light to be incident into the semiconductor laser 201; a laser driver 204 for repetitively alternating between a first oscillation interval wherein the oscillation wavelength of the semiconductor laser 201 increases continuously and a second oscillation interval wherein the oscillation wavelength decreases continuously; a transimpedance amplifier 205 for converting the output current of the photodiode 202 into a voltage, and amplifying the voltage; a signal extracting circuit 206 for differentiating twice an output voltage of the transimpedance amplifier 205; a counter 207 for counting the number of mode-hop pulses (hereinafter termed “MHP”) included in the output voltage of the signal extracting circuit 206; a calculator for calculating the distance of the object 210 and the speed of the object 210; and a display 209 for displaying the calculation results of the calculator 208.
The laser driver 204 supplies, as a pumping current, to the semiconductor laser 201, a sawtooth wave driving current that repetitively rises and falls with a constant rate of change relative to time. Doing so causes the semiconductor laser 201 to be driven so as to repetitively alternate between a first oscillation interval, wherein the oscillation wavelength increases continuously at a steady rate of change, and a second oscillation interval, wherein the oscillation wavelength decreases continuously at a steady rate of change. FIG. 23 is a diagram illustrating the oscillation wavelength of the semiconductor laser 201 over time. In FIG. 23, P1 is the first oscillation interval, P2 is the second oscillation interval, λa is the minimum value for the oscillation wavelength in each of the intervals, λb is the maximum value for the oscillation wavelength in each of the intervals, and Tt is the period of the sawtooth wave.
The laser beam emitted from the semiconductor laser 201 is focused by the lens 203, to be incident on the object 210. The light that is reflected from the object 210 is focused by the lens 203, to be incident into the semiconductor laser 201. The photodiode 202 converts the optical power of the semiconductor laser 201 into an electric current. The transimpedance amplifier 205 converts the output current of the photodiode 202 into a voltage, and amplifies the voltage, where the signal extracting circuit 206 twice differentiates the output voltage of the transimpedance amplifier 205. The counter 207 counts the number of MHPs included in the output voltage of the signal extracting circuit 206 for the first oscillation interval P1 and the second oscillation interval P2. The calculator 208 calculates the distance to the object 210 and the speed of the object 210 based on the minimum oscillation wavelength as and the maximum oscillation wavelength λb of the semiconductor laser 1, the number of MHPs during the first oscillation interval P1, and the number of MHPs during the second oscillation interval P2.
In the laser measuring device as set forth above, there are problems in that there would be errors in the numbers of MHPs counted by the counter due to, for example, noise, such as stray light, being counted as MHPs, or MHPs that are uncountable due to gaps in the signal, producing errors in the calculated physical quantities, such as distances.
Given this, the present inventor has proposed a counter that is able to exclude the effects of undercounting or overcounting at the time of counting by measuring the periods of the MHPs during the counting interval, deriving, from the measuring results, a frequency distribution of the periods during the counting interval, calculating a representative value for the period of the MHPs, from the frequency distribution, calculating, from the frequency distribution, a total Ns for the number of incidents of classes that are no more than a first specific multiple of the representative value and a total Nw of the frequencies of classes that are no less than a second specific multiple of the representative value, and then correcting the results of counting the MHPs based on these frequencies Ns and Nw. (See, Japanese Unexamined Patent Application Publication 2009-47676 (“JP '676”).)
The counter disclosed in JP '676 is able to perform generally good corrections, insofar as it does not severely reduce the SN (signal-to-noise ratio).
However, the counter disclosed in JP '676 produces chattering, due to noise caused by higher frequencies rather than the MHPs, around the binarization threshold value in the signal inputted into the counter when the signal strength in measuring at a short distance is relatively strong as compared to the width of hysteresis. In some cases there would be many short-period signal or signals with periods of about half of the actual period for the MHP. In this case, a period that is shorter than the actual period of the MHP would become the representative value of the distribution of periods. This makes it impossible to properly correct the results of counting the MHPs, and thus there was a problem in that the results of counting the MHPs would be larger, for example, a few times as large, as the actual value.